Method of producing semiconducting monocrystalline silicon on spinel substrates

ABSTRACT

THERE IS DISCLOSED AN IMPROVED METHOD OF PRODUCING MONOCRYSTALLINE SEMICONDUCTOR MATERIAL ON A SUBSTRATE OF DIELECTRIC MATERIAL. THE METHOD COMPRISES HEAT TREATING OF THE SUBSTRATE, SPECIFICALLY MGO.AL2O3 SPINEL TO MODIFY THE COMPOSITION OF ITS SURFACE AND THEN GROWING   A MONOCRYSTALLINE SEMICONDUCTOR MATERIAL, SILICON, THEREON. RESULTANT SEMICONDUCTOR DEVICES HAVE DEMONSTRATED IMPROVED SWITCHING SPEED.

Mmh'u, 1974 v. R. PORTER ETAL 3,796,597

METHOD OF PRODUCING SEMICONDUCTING MONOCRYSTALLINE SILICON ON SPINEL SUBSTRATES Find Nov. 2. 1970 2 SheetseeSheet 1 l3 fS/h'can/ \i\\ K V// Lawrence 0 Dyer ATTOR/Vf) March 12, 1974 v v. R. PORTER ETAL 3,796,597

METHOD OF PRODUCING SEMICONDUCTING MONOCRYSTALLINE SILICON ON SPINEL SUBSTRATES Filed Nov. 2, 1970 2 Sheets-Sheet I \h I i 1/ NS PE SILICON United States Patent 3,796,597 METHOD OF PRODUCING SEMICONDUCTING MONOCRYSTALLINE SILICON ON SPINEL SUBSTRATES Vernon R. Porter and Lawrence Dean Dyer, Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex.

Filed Nov. 2, 1970, Ser. No. 86,205 Int. Cl. B44d N18 US. Cl. 117-213 3 Claims ABSTRACT OF THE DISCLOSURE There is disclosed an improved method of producing monocrystalline semiconductor material on a substrate of dielectric material. The method comprises heat treating of the substrate, specifically MgO-Al O spinel to modify the composition of its surface and then growing 'a monocrystalline semiconductor material, silicon, thereon. Resultant semiconductor devices have demonstrated improved switching speed.

This invention relates to semiconductor-on-insulator (SOI) electronic device manufacture and, more specifically, to an improved method of producing monocrystalline silicon on spinel.

'The invention is directed to a method of producing single crystal semiconducting layers epitaxially on insulating substrate crystals and particularly to improvement in the art as taught in US. Pats. 3,414,434, M. Manasevit and 3,424,955, H. Seiter and C. Zaminer.

One of the problems that arises with the growth of monocrystalline semiconductor layers on variable composition insulating spinel crystals such as MgO-xAl O is that the chemical composition for optimum growth of the spinel is very closely MgO-1Al O whereas the optimum chemical composition for growth of monocrystalline silicon thereon is MgO-xAl O where x is between 1.05 and 1.1. MgO-AI O spinel is readily grown by the Czochralski technique, is crystallographically quite perfect, and has no inherent limitations upon diameter. MgO-xAl O spinel, wherein x is greater than 1, may be grown by the Verneuil or flame fusion technique, but the diameter is essentially limited to less than 1 inch by problems with cracking of the boule, and the crystalline quality is low because of grain boundaries. It is an object of this invention to provide a method utilizing the Czochralski-grown material of composition very nearly MgO-Al O and of high perfection by shifting the composition of the surface layer toward a more desirable composition with regard to deposition of the semiconductor material. It is a further object of the invention to .provide a method wherein a high degree of crystallographic perfection of the insulating surface is achieved while accomplishing the shift of composition. A further object of the invention is to provide a method achieving a greater degree of crystal quality than gained by flame fusion techniques.

The present invention may be generally described as a heat treatment of the insulating substrate in a stream of reducing gas such as hydrogen while maintaining the substrate within a relatively narrow temperature range so that MgO will be preferentially removed from the surface layer until the layer reduces an optimum composition with regard to deposition and, at the same time, any amorphous spinel material left from the polishing procedure is converted to-well-oriented spinel of high perfection. The time and temperature are controlled so that deleterious defects such as grain boundaries, twins, and high concentrations of dislocations are not permitted to arise in the spinel. Thus, when the epitaxial layer of silicon is deposited thereon, said layer similarly exhibits a high degree of perfection.

Patented Mar. 12, 1974 ice Further objects and advantages of the invention will be apparent from the following complete specification and the drawings wherein:

FIG. 1 is a cross-section of a starting substrate in accordance with the invention;

FIG. 2 is a cross-section of the substrate after heat treating;

FIG. 3 is a cross-section of the substrate with the semiconductor layer thereon;

FIG. 4 is a cross-section of the substrate with semiconductor devices diffused and isolated in the semiconductor layer;

'FIG. 5 is a standard insulated gate field effect transistor; and

FIG. 6 is an insulated gate field effect transistor from a silicon-on-insulator substrate.

Referring now to FIG. 1, there is depicted generally schematically a spinel substrate 10. This substrate 10 is a (111) slice from a boule of monocrystalline MgO-Al O grown by the Czochralski technique. This technique has been found to produce crystals which are essentially perfect. The substrate 10 may be obtained from the 'boule by any appropriate technique as, for example, sawing. 'Ihe slice is then polished to remove any marks resulting from the slicing operation. The substrate 10 therefore has surfaces which, when tested by electron diffraction techniques, are found to be essentially amorphous. The surface upon which the monocrystalline semiconductor is to be deposited must be converted to a proper crystal structure before such deposition and surface annealing or chemical etch have been previously suggested as suitable for this purpose.

However, in accordance with the invention, the substrate 10 may be placed in a suitable reactor, heat treated at a temperature between 1040 C. and 1145 C. and in a reducing hydrogen atmosphere to recrystallize the amorphous surface region and preferentially remove MgO in the surface layer 12 (FIG. 2) to produce monocrystalline MgO-xAl O wherein x, is between 1.05 and 1.1. The temperature of theheat treatment must. be controlled to assure conversion of theentire surface of the substrate 10 to the desired composition without producing deleterious defects such as cracks, twins and dislocations. These defects are found tobe created to an unacceptable level when the substrate is heated over 1145" C. At a preferred temperature-of approximately 1116 C., total conversion of the surface has been found to take place when the substrate has-been heated for a period of two hours. The heat treatment has been found to yield good results at even lower temperatures, but at approximately 1040 C., the time of treatment exceeds four hours and at 900 C., the time of treatment becomes impractical.

The substrate 10 with layer ,11 thereon is a monocrystalline semiconductor layer 13 preferably of silicon (FIG. 3). The layer 13 can be conveniently epitaxially produced as a further step in the same reactor as the heat treatment by changing the hydrogen atmosphere to a mixture of 0.3% silane in hydrogen with a phosphine dopant. The layer 13 could be produced by any appropriate epitaxial technique with or without dopant material added; From the foregoing, thereis thus provided a semiconductor-on-insulator structure suitable for the manufacture of semiconductor devices and integrated'circuits.

As depicted in FIG. 4, an array of semiconductor dioxides 15 may be produced in the epitaxial layer 13 by suitable masking and diifusion techniques as are wellknown. Etching of openings 14 through the epitaxial layer provides electric isolation between diodes.

Results of the process in accordance with the invention are given in the following examples:

Example IA slice .020 inch thick was cut from a 1% inch diameter boule of Czochralski-grown spinel crystal and polished. The crystal orientation of the slice was approximately 111) and its composition very nearly Electron diffraction by reflected high energy electrons revealed that the surface layers were essentially amorphous, due to the slicing and polishing processes. The slice was heated in hydrogen gas along with a high resistivity bulk silicon control slice on a silicon carbide coated susceptor for two hours at a temperature of 1116 C., after which a two micron silicon layer was epitaxially deposited on both samples from a .3% silane-in-hydrogen mixture at a deposition temperature of approximately 1080 C. A phosphine dopant was added to the deposition gas stream so that the silicon layer on silicon was ndoped to .15 ohm-cm. After deposition, the silicon-onspinel was again subjected to reflected high energy electron difr'raction and to X-ray measurements. The results of electron diffraction showed excellent single crystal silicon films, with no twins and a good Kikuchi diagram pattern. The resistivity across the slice was .40i.05 ohmcm. The silicon on spinel can be said to have a merit factor of as defined by Mercier in the Journal of the Electrochemical Society 117, No. 5, p. 666 (1970). The surface composition lay in the range where x was between 1.05 and 1.1.

Example II-Another MgO-Al O spinel slice Was treated as in Example I, except that the time of heat treatment was only /zhour. The electron diffraction results were the same, i.e., high quality silicon. However, only the edges showed substantial M-gO removal to produce at between 1.05 and 1.1. The average resistivity was now 1.30 ohm-cm, or a merit factor of 11.7%. Thus, although the crystalline quality was acceptable at /2 hour treatment, the composition was not.

Example IIIAnother slice was treated as in Example I, except that the time of treatment at 1116 C. was only minutes. This time the resistivity was uniform within i% over the entire slice, but its magnitude was 1.03 ohm-cm., which corresponds to a 16% merit figure. Thus, 10 minutes is too short a time for the heat treatment to achieve the crystallographic perfection of the slice in Example I.

Example IVAnother slice was treated as in Example I, except that the temperature and time of heat treatment were 1200 C. and one hour respectively. The deposited silicon had a merit factor of 28% but now showed a series of steps that would make the slice unacceptable for defect content. The defect stemmed from cracks in the silicon. Thus, a temperature of heat treatment of 1200 C. is too hot for maintaining surface quality at an acceptable level.

Example VAnother slice was treated as in Example 1, except that the temperature and time of heat treatment were 1145 C. and two hours, respectively. The deposited silicon had a merit factor of 37.6% but also showed a series of steps that would generally make the slice unacceptable for defect content. Thus, a temperature of 1145 C. is too high for maintaining surface quality with great consistency.

Example VIAnother slice was treated as in Example I, except that the temperature and time of heat treatment were 1100 C. and two hours, respectively. The deposited silicon had a merit factor of 38.1% and was free of the steps resulting in Examples IV and V. Thus, heat trelatment at 1100 C. for two hours gives acceptable resu ts.

FIGS. 5 and 6 illustrate insulated gate field eflect transistors on a standard silicon substrate and on a spinel substrate made in accordance with the invention. The standard device 20 includes a gate electrode 21 on dielectrio 22 overlying a channel 23 defined between source and drain diffusion 24, 25. Electrodes 26 and 27 make ohmic contact with the ditfusions and overlie the insulating layer 28. The inherent capacitanees in the device of this type are the lead capacitance (C between electrodes and substrate, junction capacitance (C between diffusions and substrate, and the Miller capacitance (C between gate electrode 21 and the channel 23. The insulated gate field eifect transistor 30 includes a gate electrode 31 on dielectric layer 32 overlying a channel 33 defined by a diffusion 34. The source 35 and drain 36 are of the epitaxial silicon from the foregoing process. Electrodes 37 and 38 make electrical contact with regions 35 and 36 and lie directly on the spinel substrate. With the latter construction, the lead capacitance (C becomes approximately 0 and the junction capacitance (C and Miller capacitance (C greatly reduced. A comparison of electrical characteristics is as follows:

Mobility 280 cmfl/v. see

Field effect mobility cmJ/v. sec 50-12% 1:11?{.)/v. sec. (35-70% Threshold voltage 12-14 v 1012 v. Leakage 10- A. at 10 v- =10 A. at 10 v. Capacitance 5 pi .3 pi.

Although the invention has been described and illustrated in accordance with the preferred embodiment thereof, it will be apparent to one skilled in the art that certain modifications can be made therein without departing from the spirit and scope of the invention.

What is claimed is new and desired to be secured by Letters Patent of the United States:

1. A method of producing monocrystalline semiconductor-on-insulator comprising:

(a) heat treating a substrate of MgO-Al o at 1045- 1145 C. for a time sufficient to modify the surface composition of the substrate to MgO-xAl O where x is between 1.05 and 1.1; and

(b) epitaxially depositing a thin monocrystalline layer of silicon on the modified surface.

2. A method as recited in claim 1 wherein said heating is at a temperature of 1116 C. for approximately two hours.

3. A method as recited in claim 1 wherein said heating is in a hydrogen atmosphere.

Wang, C. C. Growth and Characterization of Spinel Smgle Crystals for Substrate Use in Integrated Electroni1c;,69In J. Applied Phys. 40(9), pp. 3433-3444, August Colin, F. Phases Obtained During the Reduction of Some n-Al O -MO Oxides, In Chem. Abstracts, 70-: 109538 c (1969).

CAMERON K. WEIFFENBACH, Primary Examiner US. Cl. X.R. 117-201; l48175; 317-234 R 

